1. Field of the Invention
The invention relates to a detector to test for trace amounts of substances of interest.
2. Description of the Related Art
Terrorism risks continue at transportation facilities, government buildings, banks, restaurants, hotels and other locations where there is a significant flow of pedestrian or vehicular traffic. As a result, virtually all airports and many other buildings now include apparatus for detecting trace amounts of explosives.
Narcotics are illegal and insidious. Furthermore, it is known that many terrorist organizations fund their terrorism through the lucrative sale of narcotics. Accordingly, many airports and other public buildings recognize the need to check for narcotics.
Ion mobility spectrometers have been commercially available since about 1970, and are used to test for the presence of at least selected constituents in a stream of sample gas. An ion mobility spectrometer can be used to detect the presence of explosives in the sample gas. The typical prior art ion mobility spectrometer includes an ionization region and a drift region. A sample of air to be analyzed is fed into the ionization region on a stream of carrier gas containing a halogenated compound. The carrier gas is ionized by β particles emitted from radioactive walls of the ionization chamber to form positive ions and electrons. The electrons are captured by gases, causing a series of reactions that lead to the ionization of the halogen. Molecules of interest form ions by interaction with these gas phase ions. An electric field is established in the ionization region. The polarity of the field can be set to direct the ions of interest towards the drift region of the prior art detector. The ions travel through the drift region and towards a collector electrode at the end of the drift region opposite the ionization region. The drift time to the collector electrode varies in accordance with the size-to-change ratio of the ions. A current will be established at the collector electrode at different times depending upon the arrival times of the ions. This current is amplified and converted to a voltage for signal analysis purposes. Specific substances of interest will have a unique drift time. The detector can be calibrated to identify substances of interest based on that drift time and produce an alarm signal to the operator. Unfortunately, the conventional ion mobility spectrometer allows ions to pass into the drift region for only a short period of time. Ions arriving at the entry to the drift region at all other times are discharged. As a result, most ions are discharged in older ion mobility spectrometers and the ionization and collection efficiency of older ion mobility spectrometers is less than 0.01%. Accordingly, older ion mobility spectrometers can not detect many substances of interest that might be present in a sample.
U.S. Pat. Nos. 5,200,614 and 5,491,337 each disclose an ion mobility spectrometer (ITMS) that provide enhanced ability to detect trace amounts of substances of interest. The ion mobility spectrometer shown in U.S. Pat. No. 5,200,614, carries a sample vapor into a detector inlet on a carrier gas. The carrier gas may be doped with a low concentration vapor employed as a charge transfer mediator. Sample molecules of interest are fed through an inlet and a diffuser, and into an ionization chamber. The prior art ionization chamber has a cup-shaped metal wall and a radioactive material is disposed in the chamber. An open grid electrode is at the downstream end of the ionization chamber and normally is at the same potential as the metal walls of the ionization chamber. Thus, a largely field-free space is defined in the ionization chamber. Electrons and ion charges build up this field-free space and interact with the sample molecules under bombardment by beta-particles from the radioactive walls. However, a field is established periodically across the ionization region to sweep the ions into a drift region of the ITMS. The ions in the drift region experience a constant electric field maintained by annular electrodes, and are impelled along the drift region towards a collector electrode for analysis on the basis of their spectra. The field across the grid electrode and metal cup of the ionization chamber is reduced again to zero after about 0.2 mS and the ion population again is allowed to build up in the chamber preparatory to the imposition of the next field. The polarity of the fields of the prior art detector are chosen on the basis of whether the detector is operated in a negative or positive ion mode. A negative ion mode usually is preferred when detecting explosives. U.S. Pat. No. 5,491,337 discloses an ion mobility spectrometer with enhanced performance in a positive mode to test for the presence of trace amounts of narcotics. U.S. Pat. No. 6,765,198 discloses an ion mobility spectrometer that can analyze a single sample in a negative mode to test for explosives and in a positive mode to test for narcotics. The disclosures of U.S. Pat. Nos. 5,200,614, 5,491,337 and 6,765,198 are incorporated herein by reference.
Detectors that incorporate the teaching of U.S. Pat. Nos. 5,200,614, 5,491,337 and 6,765,198 are marketed by GE Security, Inc. and have proved to be very effective and commercially successful. However, a demand still exists for detectors with improved resolution. In this regard, it has been determined that the resolution of the peaks detected by the above-described detectors are dependent on the width of the pulse of ions introduced into the drift chamber as well as the broadening that the clouds of ions experience in the drift chamber due to diffusion, electronic repulsion and other factors. It also has been determined that the area of the outer periphery of the ionization chamber close to the grid electrode create a higher electric field during the introduction of ions into the drift chamber. Ions in these areas of higher electric fields would be introduced into the drift region before ions in lower electric field areas of the ionization chamber inwardly from the peripheral walls that define the ionization chamber. This difference in ion introduction time widens the pulse of ions and degrades the overall peak resolution of the detector. Accordingly, an object of the invention is to provide a detector with improved resolution.